One striking property of biological systems is the stereospecificity associated with many of the processes. Thus, the majority of chemical substances formed and broken down in metabolic processes are optically active and, of the two possible stereo-isomeric forms in which such substances can exist, only one is usually formed in these processes and found in natural products. In the case of the .alpha.-amino acids, e.g., only the L-members occur extensively in nature. This stereo-specificity plays an important role in the development of compounds for pharmaceutical utility. It has been found on many occasions that the physiological activity of a particular compound resides almost exclusively in one of its stereo-isomeric forms. Thus, the other stereo-isomer which may be present plays the role of an inert substance with regard to desired pharmaceutical activity, but such isomer can contribute to certain undesirable side effects. Thus, it has become of great importance to the pharmaceutical industry to provide processes for the preparation of specific optical isomers of physiologically active compounds.
The classical chemical procedure to accomplish this involves the procedure known as resolution. In this technique a racemic mixture of the compound is reacted with an optically active reagent to form a diastereomeric mixture of the substrate. These diastereomers have different physical characteristics from each other and therefore may be separated by conventional procedures such as fractional crystallization, distillation, etc. One of the diastereomers may then be converted to the desired optical isomer of the starting compound by conventional techniques such as hydrolysis. This procedure suffers from the obvious disadvantage of yielding only a theoretical maximum of 50 percent of the desired stereo-isomer based on the racemic starting material. Due to the number of reaction and purification steps involved, practical yields are substantially lower than this figure.
An alternate procedure employs reagents of biological origin such as enzymes which possess many asymmetric centers and hence are themselves highly asymmetric. In such procedure the racemic mixture is treated with an enzyme which will interact with only one of the two optical isomers of the substrate. In this manner either the undesired component is converted into a different derivative which may then be separated from the desired isomer of the initial compound, or alternatively the desired compound may interact with the enzyme to form a derivative which may be isolated by conventional purification techniques. Again such procedures suffer from the theoretical disadvantage of yielding a maximum of 50 percent of the desired optically active compound.